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Researchers discover reasons for red leaves in fall

By Chantaie AllickStaff Reporter

Sat., Oct. 8, 2011

It’s hard to miss signs of autumn in Ontario. It’s often a kaleidoscope of oranges, greens, browns, yellows and reds as summer shifts to winter and verdant foliage makes way for the coming cold.

Just in time for this year’s senescence, the controlled process of nutrient transfer or leaf death, researchers have answered a question that has stumped scientists for years: why some leaves turn red and not yellow or orange.

It’s long been known — at least among the experts — that leaves turn yellow and orange as they die and chlorophyll levels in them lower with the temperature.

But plants also produce a pigment called anthocyanin at this time of year, according to a Sept. 29 article in New Scientist Magazine. Anthocyanin turns the leaves red. The plants expend energy to produce the pigment, which normally wouldn’t make sense for them to do as they prepare for winter hibernation. Over the past few years, researchers have finally figured out why.

“There’s a lot of excitement among geologists about this and the biologists are cautiously optimistic,” said geologist Martha Eppes, who made discoveries that contributed to the understanding. “It’s certainly been a mystery for centuries.”

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William Hoch of Montana State University laid the foundation from the biological side of things. In a 2003 study, he and his colleagues found that if they blocked the pigment’s production, leaves became vulnerable to sunlight. The leaves in turn sent fewer nutrients to roots for winter storage. In other words the leaves turn red in the process of transferring nutrients to tree roots in preparation for winter.

Meanwhile, a series of studies by professor Eppes and her students at the University of North Carolina found that trees in nutrient-poor soils produce the pigment. They looked at the situation from the ground up. A student noticed that redder leaves were produced on hill slopes near the school and yellow leaves down in the plains.

From that observation they developed an experiment that found the pigment-producing trees survive better higher up on hills than non-anthocyanin producing trees. It’s Darwinian in the sense that the ones able to produce anthocyanin are also the ones that can survive in less nutrient-rich environs.

“The pigment-producing trees are able to outcompete the non-pigment producing trees,” said Eppes in a phone interview. Something about the pigment allows the trees to better thrive in nutrient-poor soils. A least that’s the hypothesis her work is based on. She says there’s a strong correlation between soil nutrients and anthocyanins in leaves.

From Hoch’s perspective it’s more a matter of “photosynthetic capacity,” or a tree’s ability to turn light into energy efficiently. In contrast to Eppes, he looks up at the leaves and what they do.

Hoch says pioneer tree species like paper birch, with its peeling white bark — the ones that don’t produce anthocyanin — have a better capacity for photosynthesis than do their pigment producing cousins. So when the process slows down and trees begin to break down their leaves, pioneer species are more stable. They don’t need the anthocyanin.

“Nature always finds different ways of doing things,” said Hoch. It’s akin to pollination and the myriad ways in which plants are pollinated in nature, he explains. Trees simply have many ways of transitioning from summer to winter.

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Simch Lev-Yadun, a researcher at the University of Haifa in Israel, and Jarmo Hopainen at the University of Eastern Finland, went back to the ice age for their part in solving the riddle.

The pigment, also an antioxidant, protects against damage caused by insects. They say this may be why autumn leaves are more often red in the United States and yellow in Europe.

Insects that attacked deciduous trees before and between ice ages in North America mostly died out during the European ice ages. Trees in Europe didn’t need the added protection of anthocyanin.

Together the researchers’ work helps answer a question that biologists have been working on for more than a century.

Next Hoch will attempt to understand the mechanics and genetics behind the process.

For those simply interested in seeing more red this month, Michelle Bourdeau with LEAF in Toronto says sugar maples, trees with leaves much like the one on the Canadian flags, are your best bet for seeing bright red leaves this fall.

They’re found mostly in ravine areas and parks. The higher up on a slope trees are, at least according to Eppes’ research, the more likely it is that they’ll have red leaves in the coming weeks. So those parked on the Don Valley Parkway during rush hour need simply look up out of the valley to see the red among the green, oranges and yellows that will populate the province this fall.

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